1. Field of the Invention
This invention relates to a printing method in an ink jet recording apparatus.
2. Related Background Art
With the spread of copying apparatuses, information processing instruments such as word processors and computers, and further, communication instruments, there is going on the rapid spread of apparatus which effect digital image recording by the use of a recording head of the ink jet type as image forming (recording) apparatuses for those instruments. Further, with the tendency of the information instruments and communication instruments toward multicoloring and lower costs, the demand for recording apparatuses coping with multicoloring and printing on plain paper has been increasing. Among such recording apparatuses, popular ones are those which, for the improvement in recording speed, use, as a recording head comprising a plurality of recording elements integrated and arranged (hereinafter referred to as the multihead), a head in which a plurality of discharge ports and a plurality of liquid paths communicating therewith are integrated as ink discharge portions and which is provided with a common liquid chamber for temporarily storing therein ink to be supplied to each of the discharge portions, and which are provided with a plurality of such multiheads to cope with colors.
FIG. 5 of the accompanying drawings shows the construction of a printer unit when printing is effected on paper by the multihead. In this figure, the reference numeral 501 designates ink cartridges. These ink cartridges are comprised of ink tanks filled with inks of four colors, i.e., black, cyan, magenta and yellow, respectively, and a multihead 502. FIG. 6 of the accompanying drawings shows the manner in which multinozzles are arranged on the multihead from a direction z, and in FIG. 6, the reference numeral 601 denotes multinozzles arranged on the multihead 602. Turning back to FIG. 5, the reference numeral 503 designates a paper feeding roller which is rotated in the direction of arrow to feed printing paper 507 in a direction y while holding down the printing paper 507 with an auxiliary roller 504. The reference numeral 505 denotes a paper supply roller which effects the supply of the printing paper and also performs the function of holding down the printing paper 507, like the rollers 503 and 504. The reference numeral 506 designates a carriage for supporting the four ink cartridges and moving these cartridges with printing. This carriage 506 is designed to stand by at a home position (h) indicated by dotted line when printing is not being effected or when the recovery operation or the like of the multihead is effected.
The carriage 506, which is at the home position before printing is started, effects printing on the paper over a width D by n multinozzles 601 on the multihead 502 while being moved in the direction x when a printing starting command comes to it. When the printing of data is terminated to the end portion of the paper, the carriage is returned to the home position, and again effects printing in the direction x. By the time when the second printing is started after the first printing has been terminated, the paper feeding roller 503 is rotated in the direction of arrow, whereby the paper is fed by the width D in the direction y. In this manner, printing and paper feeding by the multihead width D are repeated during each scan of the carriage, whereby the printing of data on the paper is completed.
In such a printer coping with colors, it is usual that the ink discharge amount per dot is designed uniformly for all colors in order to keep color balance. Further, when printing is effected singly with each color to cope with the blur in the boundary portion between different colors which poses a problem during plain paper printing, it is sometimes the case that printing is completed in a state in which the area factor cannot satisfy 100% even when in a predetermined area, discharge is effected with printed data of 100% duty (solid). However, such designing of the discharge amount is based on the color image data and therefore, where the apparatus is used as an ordinary monochrome printer, density unavoidably becomes low on characters and ruled lines, and this has led to a problem that the print lacks in clarity.
In order to solve this problem, there has been proposed a method whereby the multihead is scanned twice each for the same area and in the second scan, black alone is printed. If this is done, only the black ink of an amount double that in the ordinary case is impacted and therefore, a corresponding increase in density becomes possible.
In this method, however, the multihead must be scanned to the same position twice each to emphasize the black position and thus, time cost double that for ordinary printing will be required.
So, in order to further speed up this emphasis of black, applicant proposed a method of printing inks of the other colors at the black image data position during the same scan as that for black (U.S. patent application Ser. No. 685,210). If this is done, the black dye of an amount several times as great as that is the ordinary case can be provided at the same impact point by only one scan.
In this manner, it has been possible to emphasize the density of black at the same time cost as that in the ordinary case.
However, if the above-described method is intactly used, the area factor will exceed 100%, but the discharge amounts of the other colors will become great as compared with the discharge amount of black and therefore, the hue of color will become different from black. Also, if the above-described method is intactly used for plain paper, the great problem of the "blur in the boundary portion between different colors" encountered when coping with plain paper will tend to the further aggravated. This problem depends on the amount of ink impacted against a predetermined area at one time. Accordingly, if as in the above-described method, an amount of ink several times as great as that during ordinary printing is shot on the same impact point at the same time, the ink which cannot be absorbed in a vertical direction on plain paper will expand in a horizontal direction and go to blur in the area to be printed with the other colors.